The present invention relates to an immobilizing device particularly useful during the processing and/or testing of planar workpieces on a flat surface of a table. The invention is especially useful in the testing of printed circuit boards, and is therefore described below with respect to this application.
Printed circuit boards, particularly those carrying complex conductive pathways having critical dimensions, must be carefully tested before they are incorporated in the electronic equipment for which they are designed. At the present time, testing of the printed circuit boards is usually carried out on equipment which is largely automated and computerized. Such equipment generally includes a movable table for receiving the board, and electro-optic devices for scanning the board in order to locate defects. Reliable scanning requires proper mounting of the board on the table surface. Thus, the board must not only be perfectly immobilized relative to the table surface, but the board surface to be checked must be flat and parallel to the table surface in order for all details to be within the depth of focus of the optic system.
Because of the relative thinness and flexibility of printed circuit boards commonly used today, the foregoing requirements mean that the boards must be positively pressed against the table surface. The various immobilizing devices in use today for pressing the board against the table surface include: suction, spring-loaded rollers, pressure pads, relatively heavy glass plates placed on the board, and adhesive tape. However, none of these known arrangements is entirely satisfactory because they introduce handling or optic system difficulties, increase the test time, and/or reduce the inspection accuracy.